1. Field of the Invention
The present invention generally relates to wireless communication systems and, more particularly, to enhancing the functionality and enabling features of reference signals transmitted from a User Equipment. The reference signals provide an estimate of the channel medium experienced by the User Equipment at a given time instance.
2. Description of the Related Art
Several types of signals need to be supported for the proper functionality of a communication system. In addition to data signals, conveying the information content, control signals also need to be transmitted from User Equipments (UEs) to their serving Base Station (BS or NodeB) in the UpLink (UL) of the communication system and from the serving NodeB to the UEs in the Downlink (DL) of the communication system to enable proper processing of the data signals. For example, control signals include positive or negative ACKnowledgement signals (ACK or NACK, respectively) that are transmitted in response to (correct or incorrect, respectively) data packet reception and are associated with a Hybrid Automatic Repeat reQuest process (HARQ-ACK signals). Control signals also include Channel Quality Indication (CQI) signals, which a UE sends to the NodeB to provide information about the DL channel conditions that the UE experiences. Further, Reference Signals (RS), also known as pilot signals, are typically transmitted to provide channel estimation and enable coherent demodulation for the transmitted data or control signals or, in the UL, to be used by the receiving NodeB to measure the UL channel conditions the UE experiences. The former RS used for demodulation of data or control signals will be referred to as DeModulation RS (DMRS) while the latter RS, which are typically wideband in nature, are used for sounding the UL channel medium and will be referred to as Sounding RS (SRS).
A UE, also commonly referred to as a terminal or a Mobile Station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, and the like. A NodeB is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or similar terminology.
UEs transmit data or control information through a Physical Uplink Shared CHannel (PUSCH) while, in the absence of PUSCH transmission, the UEs transmit control information through a Physical Uplink Control CHannel (PUCCH). A UE receives the signals conveying data information through a Physical Downlink Shared CHannel (PDSCH) while signals conveying control information are received through a Physical Downlink Control CHannel (PDCCH).
A UE is assumed to transmit in the PUSCH or in the PUCCH over a Transmission Time Interval (TTI) which may, for example, correspond to a sub-frame with a duration of 1 millisecond (msec). FIG. 1 illustrates a block diagram of a sub-frame structure 110 for PUSCH transmission. The sub-frame includes two slots. Each slot 120 includes seven symbols. Each symbol 130 further includes a Cyclic Prefix (CP) in order to mitigate interference due to channel propagation effects. Some symbols in each slot may be used for the DMRS transmission 140. The second DMRS in the sub-frame may or may not be transmitted with its negative value (scaled with “−1”) 150 as it is subsequently described. The PUSCH transmission BandWidth (BW) consists of frequency resource units which will be referred to as Resource Blocks (RBs). In one example, each RB includes NscRB=12 sub-carriers which are also referred to as Resource Elements (REs). A UE may be allocated one or more RBs 160 for PUSCH transmission and one RB for PUCCH transmission.
PUSCH transmission or PDSCH reception by a UE may be scheduled by the NodeB either dynamically, through a respective Scheduling Assignment (SA) conveying a Downlink Control Information (DCI) format in a PDCCH, or through Semi-Persistent Scheduling (SPS) using UE-specific higher layer signaling such as Radio Resource Control (RRC) signaling. The DCI format may inform a UE about a data packet transmission by the NodeB in the PDSCH (DL SA) or about a data packet transmission to the NodeB (UL SA) in the PUSCH. With SPS, a UE transmits or receives data packets at predetermined sub-frames.
FIG. 2 illustrates a processing chain at the NodeB for a SA transmission. The Media Access Control (MAC) layer IDentity of the UE (UE ID) for which the SA is intended for masks the Cyclic Redundancy Check (CRC) of the SA information bits in order to enable the UE to identify that the SA is intended for it. The CRC 220 of the SA information bits 210 is computed and then masked 230 using the eXclusive OR (XOR) operation between CRC bits and UE ID bits 240. An XOR operation only evaluates to true where only one of the two input bits is 1. Thus, XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC is then appended to the SA information bits 250, channel coding (such as convolutional coding) is performed 260, followed by rate matching 270 to the allocated PDCCH resources, and finally by interleaving and modulation 280, and transmission of the SA 290. It is assumed that both the CRC and the UE ID have the same length such as, for example, 16 bits.
The UE receiver performs the reverse operations of the NodeB transmitter. This is illustrated in FIG. 3. The received control signal 310 is demodulated and the resulting bits are de-interleaved 320, the rate matching applied at the NodeB transmitter is restored 330 and then followed by decoding 340. The SA bits 360 are then obtained after extracting the CRC bits 350, which are then de-masked 370 by applying the XOR operation with the UE ID 380. Finally, the UE performs the CRC check 390. If the CRC check passes, the UE considers the SA as a valid one and determines the parameters for signal reception (DL SA) or signal transmission (UL SA). If the CRC check does not pass, the UE disregards the presumed SA.
The DMRS is assumed to be generated from Constant Amplitude Zero Auto-Correlation (CAZAC) sequences. An example of such a sequence is given by the following Equation (1):
                                          c            k                    ⁡                      (            n            )                          =                  exp          [                                                    j                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                k                            L                        ⁢                          (                              n                +                                  n                  ⁢                                                            n                      +                      1                                        2                                                              )                                ]                                    Eq        .                                  ⁢                  (          1          )                    where L is a length of the CAZAC sequence, n is an index of a sequence element, n={0, 1, 2, . . . , L−1}, and k is a sequence index. For CAZAC sequences of length L, with L being a prime number, the number of sequences is L−1. Therefore, an entire family of sequences is defined as k ranges in {0, 1, 2, . . . , L−1}. However, the sequences for DMRS transmission need not be generated by strictly using the above expression. As one RB is assumed to include NscRB=12 REs, CAZAC-based sequences can be generated either by truncating a longer prime length (such as length 13) CAZAC sequence or by extending a shorter prime length (such as length 11) CAZAC sequence by repeating its first element(s) at the end (i.e., cyclic extension), although the resulting sequences do not strictly fulfill the definition of a CAZAC sequence. Alternatively, CAZAC sequences can be generated through a computer search for sequences satisfying the CAZAC properties.
FIG. 4 shows an DMRS transmitter structure at a UE based on a CAZAC sequence. The frequency domain version of a CAZAC sequence may be obtained by applying a Discrete Fourier Transform (DFT) to its time domain version. The frequency domain CAZAC-based sequence 410 is generated, the REs 420 in the assigned PUSCH transmission BW are selected 430, the Inverse Fast Fourier Transform (IFFT) is performed 440, the Cyclic Shift (CS) 450 is applied, and, finally, the CP 460 and filtering 470 are applied to the transmitted signal 480. The UE also applies zero padding in REs where the DMRS is not transmitted, such as in REs used for signal transmission from another UE (not shown). The PUSCH transmission BW may be contiguous, in accordance with the SC-FDMA transmission principle, or non-contiguous in accordance with the DFT-Spread-OFDM (DFT-S-OFDM) transmission principle. For brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas, as they are known in the art, are not shown.
The NodeB receiver performs the reverse functions of the UE transmitter. This is illustrated in FIG. 5 where the reverse operations of those in FIG. 4 are performed. In FIG. 5, an antenna receives the Radio-Frequency (RF) analog signal and after passing further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the resulting digital received signal 510 passes through a time windowing unit 520 and the CP is removed 530. Subsequently, the CS of the transmitted CAZAC-based sequence is restored 540, a Fast Fourier Transform (FFT) 550 is applied, the selection 560 of the transmitted REs 565 is performed, and correlation 570 with the CAZAC-based sequence replica 580 is applied. The resulting output 590 can then be passed to a channel estimation unit, such as a time-frequency interpolator.
In addition to the DMRS transmission, the transmission from a UE of control signals or RS in the PUCCH and their reception by the NodeB may also be based on CAZAC sequences and be respectively performed as previously described.
Different CSs of a CAZAC sequence provide orthogonal sequences. Therefore, for a given CAZAC sequence, different CSs can be allocated to different UEs and achieve orthogonal DMRS multiplexing in the same RBs. This principle is illustrated in FIG. 6. In order for the multiple CAZAC sequences 610, 630, 650, and 670 generated from multiple corresponding CSs 620, 640, 660, and 680 of the same CAZAC sequence to be orthogonal, the CS value 690 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If Ts is the duration of one sub-frame symbol, the number of CSs is equal to └Ts/D┘ where └ ┘ denotes the “floor” operation which rounds a number down to its lower integer.
For a PUSCH transmission associated with an UL SA, the UL SA is assumed to include a Cyclic Shift Indicator (CSI) indicating the CS for the CAZAC sequence used as DMRS. For SPS PUSCH transmissions, the NodeB also provides, through higher layer signaling, to the UE the CSI value. Table 1 shows an mapping of CSI values to CS values.
TABLE 1Mapping of CSI Values to CS Values.CSI ValueCS Value000CS0 = 0001CS1 = 6010CS2 = 3011CS3 = 4100CS4 = 2101CS5 = 8110 CS6 = 10111CS7 = 9
CAZAC-based sequences of the same length typically have low cross-correlations which is important for minimizing mutual interference. CAZAC-based sequences of different lengths have a wide distribution of cross-correlation values and large values often occur. FIG. 7 shows the Cumulative Density Function (CDF) of cross-correlation values for length-12 CAZAC-based sequences resulting from cyclically extending a length-11 Zadoff-Chu (ZC) sequence, truncating a length-13 ZC sequence, or computer generation of length-12 CAZAC sequences. Variations in cross-correlation values are observed and even larger cross-correlation values may occur between CAZAC-based sequences of different lengths. Randomization of the occurrence of large cross-correlations can be achieved by sequence hopping where the sequence is selected from a predetermined set of sequences according to a hopping pattern such, as for example, a pseudo-random pattern having the slot number as one of its arguments.
Sequence hopping is among CAZAC-based sequences of the same length that belong either to the same group or to different groups. A group of CAZAC-based sequences consists of sequences with different lengths, each corresponding to each of the possible PUSCH RB allocations. For example, if 30 CAZAC-based sequences exist for the minimum allocation of 1 RB and since the number of available CAZAC-based sequences increases as the number of RBs increases, 30 sequence groups can always be generated. For large RB allocations, such as at least 6 RBs, 2 sequences can be included in each group of CAZAC-based sequences. Sequence hopping among sequences in different groups will be referred to as group sequence hopping while sequence hopping among sequences in the same group (for allocations of at least 6 RBs) will just be referred to as sequence hopping. Group sequence hopping and sequence hopping (within the same sequence group) are respectively enabled or disabled by the NodeB for all UEs in its cell and for all applicable signals using transmission of sequences (DMRS in the PUSCH or control signals and RS in the PUCCH) through broadcast signaling of the respective (cell-specific) parameters: Group-hopping-enabled and Sequence-hopping-enabled.
Multi-User Multiple-Input Multiple-Output (MU-MIMO) can improve the spectral efficiency of a communication system. With MU-MIMO, PUSCH transmissions from multiple UEs share at least part of a BW. MU-MIMO is facilitated if the NodeB can obtain interference-free estimates of the channel medium the MU-MIMO UEs experience. This requires orthogonal reception for the respective DMRS. If the PUSCH transmissions from MU-MIMO UEs share exactly the same BW, orthogonal DMRS multiplexing can be obtained using different CS of the same CAZAC sequence. However, if the PUSCH transmissions from MU-MIMO UEs do not share exactly the same BW, orthogonal DMRS multiplexing using different CS is not possible because the respective CAZAC-based sequences have different lengths. Orthogonal Covering Codes (OCC) can then be used to provide orthogonal DMRS multiplexing in the time domain. For the sub-frame structure in FIG. 1 which has 2 DMRS symbols, the OCCs can be {1, 1} and {1, −1}. Regarding the CS, the OCC should also be indicated for the DMRS transmission in the PUSCH.
Two classes of UEs are assumed to coexist in the communication system. The first class of UEs, referred to as legacy-UEs, do not support OCC and rely only on the CS for orthogonal DMRS multiplexing. The second class of UEs, referred to as Advanced-UEs, support OCC and can rely on both OCC and CS for orthogonal DMRS multiplexing.
A required restriction for the application of OCC is the absence of sequence hopping Because of the sub-frame structure in FIG. 1, time-domain orthogonality is not possible if the DMRS transmission in each sub-frame slot uses a different CAZAC sequence. Therefore, although OCC is only needed by Advanced-UEs for MU-MIMO transmissions over different BWs, the performance of all UEs is degraded by the requirement to disable sequence hopping over the entire cell. Moreover, as sequence planning to achieve low cross-correlations is typically impractical, PUCCH transmissions, which are assumed to rely entirely on CAZAC sequences, and occur over only one RB are particularly impacted which is highly undesirable given that control information has enhanced reliability requirements.
Therefore, there is a need to define a mapping of CSI values to OCC and CS values that optimizes DMRS multiplexing among Advanced-UEs and among legacy-UEs and Advanced-UEs.
There is also need to enable sequence hopping in a cell while supporting time-domain orthogonality through the application of OCC for the DMRS transmission in the PUSCH.
Finally, there is need to separate the application of sequence hopping between sequences used in PUSCH transmissions and sequences used in PUCCH transmissions.